The purpose of this research is to evaluate the stress response of aging female rats during lactation. A series of studies is planned which compares the pituitary-adrenal and behavioral responsiveness of young and middle-aged rat mothers to physical, psychological, or social stress. Exposure to ether or different levels of novelty will serve, respectively, as the physical and psychological stressors, and both lactating and nonlactating females of the two age groups will be used. The social stress tests, which will utilize only lactating females, involves pup-related stimuli; a litter will be either shocked or handled before being returned to the mother. After each of these conditions, blood samples will be obtained for plasma corticosterone analysis. In addition, during the psychological and social stress tests behavioral measures will also be recorded. These experiments will therefore examine whether aging modifies the selective responsiveness that young lactating females exhibit to different types of stress situations.